1. Field of the Invention
The present invention relates to a picture taking tube having a bias light device.
It is known that the remanence performances of picture taking tubes are improved if a degree of illumination constant in time and of a low level is superimposed on the useful illumination of the photosensitive target of the tube, that is to say a degree of illumination whose level is about a hundredth of that of the maximum light signal which the tube can record. This is currently called "bias light".
It will be recalled that remanence is the speed of response of the video signal to a modification of the illumination level. In the case of a tube with photosensitive target, of the photoconducting type, the remanence depends on many factors. We may however consider that it behaves like an RC electric circuit, in which R is the resistance of the electron beam which scans the target and C is the charge storage capacity of the target. The resistance R of the beam comes from the dispersion of energy .DELTA.E between the electrons of the beam. In fact, the beam reaches the target at an average zero speed since the cathode is at zero potential. Thus, when the target is not illuminated, the beam brings the potential of its surface back to a slightly negative value -.DELTA.E/2. A small part of the beam is accepted by the target and the rest is sent back to the gun. The resistance R therefore increases progressively as the beam causes the potential of the target to be lowered towards -.DELTA.E/2.
The principle of bias light is to raise the potential of the target to a slightly positive value under the effect of an additional illumination.
Calculation and experience show that it is sufficient to raise the potential of the target to +.DELTA.E/2.
2. Description of the Prior Art
In the prior art three ways are known of providing bias light for picture taking tubes.
This bias light may be provided, not at the level of the picture taking tube and of the coil block which generally surrounds it, but at the level of the camera of which the tube forms part. The bias light device forms then part of the optics of the camera and the light reaches the front face of the photosensitive target of the tube, that is to say the face of the target turned outwardly of the tube.
This bias light ma also be achieved by acting on the picture taking tube. A part at least of the bias device is then internal to the tube. Thus, bias light devices are known in which an illumination source is used external to the tube and where glass tubes transmit the light to the inside of the tube as far as the photosensitive target. This is the case for example in the U.S. Pat. No. 3,751,703. Bias light devices are also known in which the illumination source is also internal to the tube and is formed, for example, by the filament of the cathode of the electron gun of the tube.
A last possibility is to provide the bias light at the level of the assembly formed by the coil block and mechanical covering which surround the tube and without acting on the tube itself. Thus, in the U.S. Pat. No. 4,031,551, the bias light is provided by illuminating the input glass face of the tube, laterally, that is to say in the direction of its thickness and perpendicularly to the longitudinal axis of the tube. This solution requires a coil block and a tube head (input face and adjacent parts) specially adapted for the available space is small.